In known harvesters, two crane types, with a different main principle, are generally used, which are a path-of-motion crane and a sliding-boom crane. In harvesters, path-of-motion cranes are the most generally used and are manufactured in several different implementations by several different manufacturers. The basic idea of a path-of-motion crane is to implement an essentially horizontal, approximately linear movement of the outer end of the crane's boom, and simultaneously the load being carried by it, by guiding a single operating device, for example a hydraulic cylinder. This property is regarded as being advantageous and desirable in harvester machines, the task of the boom of which is mainly to use the boom to lift trees from around the machine for processing in the harvester head.
U.S. Pat. No. 7,523,834 B2, which discloses one form of implementation for a path-of-motion crane, is known from the prior art. The path-of-motion crane consists of a base, to which is pivoted a main boom, an articulated boom being pivoted in turn to the main boom. Between the base and the main boom is a lifting cylinder for lifting the main boom and in connection with the main boom is an actuator cylinder for operating the articulated boom. The folding movement of the articulated boom is created with the aid of the actuator cylinder and an arm mechanism connected to it. Drawbacks with the solution are the additional weight brought by the arm mechanism, as well as the complexity of the design of the arm mechanism. In addition, the complicated arm mechanism, located far from the lifting boom, can obscure visibility from the operator.
According to a second solution according to the prior art, the lifting cylinder and the actuator cylinder are synchronized with the aid of a pressure cylinder, which replaces the arm mechanism. The piston rods of the pressure cylinder and the actuator cylinder are permanently connected to each other, so that, when the actuator cylinder moves, the pressure cylinder follows the movement of the actuator cylinder, producing pressure and volume flow for the lifting cylinder.
The pressure cylinder and the actuator cylinder are pivoted in parallel between the main boom and the articulated boom. The feed pressure is directed only to the actuator cylinder, which creates an uneven loading in the attachment of the actuator cylinder and the pressure cylinder. This uneven loading tends to twist the booms and the pivots, creating asymmetrical stresses in the structure of the path-of-motion crane. In order to ensure the working life of the structures, the booms and pivots must be reinforced and made sturdier than usual. In addition, the strokes of the parallel cylinders must be made very precisely the same length, otherwise the difference in the length of stroke will also create serious additional stresses in the structures.